222         CONTRIBUTIONS    TO    ECONOMIC   GEOLOGY,   1907,   PART   II. 
to  the  north  and  south.  The  relief  on  the  north  side  of  the  mountains 
is  much  greater  and  more  rugged  than  on  the  south  side.  Extending 
southeastward  from  a  point  near  the  center  of  Green  Mountains  to 
the  valley  of  Separation  Creek,  in  T.  23  N.,  R.  89  W.,  is  a  prominent 
escarpment  which  makes  up  the  fourth  district.  The  face  of  the 
escarpment  is  somewhat  irregular  and  is  cut  in  several  places  by  large 
valleys  heading  behind  the  main  crest.  The  top  of  the  escarpment 
decreases  in  altitude  gradually  toward  the  south.  From,  the  crest 
low  hills  and  broad  undulating  plains  slope  off  gently  toward  the 
southwest,  where  they  merge  into  the  flats  of  the  Great  Divide  Basin. 
The  general  character  of  the  drainage,  the  absence  of  permanent 
streams,  and  the  highly  alkaline  character  of  the  water  of  the  lakes 
of  this  area  are  unfavorable  to  economic  development  of  the  coals. 
All  but  about  300  square  miles  of  the  region  covered  by  this  survey 
has  no  outward  drainage.  The  Continental  Divide  to  the  south  of 
this  field  follows  the  crest  of  the  Sierra  Madre  and  the  Savery  Plateau 
nearly  to  the  headwaters  of  Separation  and  Muddy  creeks,  in 
sec.  7,  T.  18  N.,  R.  89  W.  Here  the  main  divide  is  replaced  by  two 
low  divides  which  separate  the  interior  drainage  from  the  Atlantic 
drainage  on  the  east  and  north  and  from  the  Pacific  drainage  on  the 
south  and  west.  The  eastern  divide  trends  east  of  north,  crossing 
the  Union  Pacific  Railroad  near  Ferris,  thence  passing  across  the 
center  of  the  Rawlins  Hills  to  a  point  near  North  Platte  River,  in 
T.  22  N.,  R.  86  W.,  thence  north  along  the  Haystack  Hills  nearly  to 
the  eastern  summit  of  the  Ferris  Mountains,  thence  nearly  westward 
to  South  Pass,  touching  the  crests  of  the  Green  Mountains,  Crooks 
Mountain,  and  Antelope  Hills  only  for  short  distances  and  making 
wide  detours  to  the  southward  around  the  drainage  basins  of  Muddy, 
Crooks  and  Alkali  creeks.  The  western  divide  runs  nearly  westward 
to  Robinson,  on  the  Union  Pacific  Railroad,  following  for  a  part  of 
the  distance  the  crest  of  the  Laney  Rim,  thence  running  northwest- 
ward to  the  vicinity  of  Steamboat  Mountain  and  South  Pass.  At 
South  Pass  the  two  low  divides  reunite  in  one  main  divide  which 
continues  northward  along  the  crest  of  the  Wind  River  Mountains. 
These  low  divides  inclose  a  basin  area  of  approximately  4,200  square 
miles. 
All  the  streams  of  the  basin  are  intermittent,  flowing  only  during 
the  melting  of  snows  in  the  spring.  They  flow  into  broad  depressions, 
forming  shallow  lakes,  many  of  which  dry  up  during  the  summer. 
Six  large  depressions  receive  most  of  the  drainage.  They  are  as 
follows,  enumerated  from  east  to  west:  Separation  flats,  in  Tps.  23 
and  24  N.,  Rs.  87  and  88  W.,  and  in  Tps.  20  to  23  N.,  Rs.  89  and  90 
W.;  Lost  Soldier  flats,  in  T.  26  N.,  R.  89  W. ;  Dry  Lake  flats,  which 
extend  westward  in  T.  22  N.  through  Rs.  91  to  94  W. ;  Chain  Lake 
flats,  which  extend  westward  through  T.  23  N.,  Rs.  90  to  94  W.; 
